This invention relates to an integrated interface circuit for driving a subscriber""s telephone line, being of the type which includes at least one output stage connected to the telephone line, and specifically to driving wide-band telephone lines and transmitting digital data at a very high frequency.
Embodiments of the invention relate to a monolithically integrated circuit for driving wide-band telephone lines and transmitting digital data at a very high frequency. The circuit is powered from a battery DC supply providing a pair of voltage references and comprises an output circuit portion including a pair of differential output stages. Each stage has a pair of inputs and being connected with its output to a respective lead of a two-wire telephone line.
This invention is applicable, in particular, to interface telephone circuits for driving the lines linked to subscribers"" phone sets, but nothing in this description should limit the scope of the invention or the claims to only that field.
These interface circuits, e.g., of the type known as SLICs (Subscriber Line Interface Circuits) in the trade, are connected between the subscriber""s telephone lines and the exchange internal circuitry, and are adapted to supply the telephone line with a predetermined line voltage and current dependent on the line resistive load.
There exists a demand in this field for telephone line communications of improved quality, as well as for a wider range of subscriber services. For example, services are currently being offered which involve transmission of digital information over the telephone lines.
Continued improvement in the quality of telephone line transmission is needed to fill such demands involving increased sophistication.
Since the inception of Internet and other digital communications facilities utilizing the telephone line pair for data transmission, it became necessary to remove current restrictions on the rate of data transmission using 64 kbits/sec speech modems. New transmission standards are being developed to improve the transmission rate without forfeiting compatibility with current telephone apparatus.
A dawning standard in the market is that known as ADSL (Asymmetric Digital Subscriber Line), which provides for the transmission at 4.3125 Hz spacings of carriers having an analog band upper limit of 1.1 MHz. These signals are added, through appropriate electromechanical circuits (transformers, capacitors, inductors, and resistors) to the standard telephone signal, using a so-called POTS splitting technique.
Stated otherwise, the POTS (Plain Old Telephone Set) function is served by conventional integrated circuits of the SLIC type, as described in this Applicant""s U.S. Pat. Nos. 5,440,612 and 5,612,998, for example.
On the other hand, the ADSL function is made possible by circuits of a new design which are adapted to transmit, receive and decode this type of a signal.
Furthermore, there exists a subset of the ADSL specifications, referred to as ADSL_Lite, which being somewhat inferior in performance, and less expensive for the user, allows data to be transmitted at a rate of 1.5 Mbits/sec in a 550 kHz analog band. The ADSL_Lite specification is presently a subject for debate, and the service providers anticipate POTS splitting as well as other solutions.
In particular, some manufacturers plan to adopt a solution derived from the solid state electronic circuits of the SLIC type as currently employed on the subscriber boards to have the speech signals and ADSL signals added together upstream of the line drive circuit portion.
However, this solution does not seem practicable with conventional line drive portions, since these involve handling the high-frequency signals by the same circuits which are to handle the necessarily high voltages (about 160V) used for the line supply and the call signals.
In fact, current technologies are inadequate to provide elementary components which can be operated at high voltages and good rates. Consequently, the objectives of the ADSL_Lite specification cannot be achieved without a POTS splitting function.
Embodiments of the invention provide an integrated circuit with suitable structural and functional features to drive a two-wire telephone line for transmitting digital data at a very high rate, thereby overcoming the drawbacks with which prior solutions are beset.
Therefore, embodiments of the invention supply to the pair of final amplifier stages connected to a respective lead of the two-wire line, a respective signal in phase opposition, so as to obtain at the output an AC+DC differential voltage for driving the telephone line. Thus, the two outputs of the driver circuit are coupled to the input signals.
Based on this principle, presented is a device for deriving a reference voltage from the supply voltage, and including a low-voltage supply network being input a DC signal and producing a voltage reference to be added to said reference voltage for delivery to one input of each output stage, the other input of each stage receiving an AC signal in order to present at the circuit output a suitable AC+DC differential voltage for driving the telephone line.